Cathodoluminescence of Y2O3:Ln3+ (Ln = Tb, Er and Tm) and Y2O3:Bi3+ nanocrystalline particles at 200 keV

Abstract

The cathodoluminescence (CL) spectra of nanocrystalline Y₂O₃:Tb³⁺ (0.3%), Y₂O₃:Er³⁺ (1%), Y₂O₃:Tm³⁺ (2%) and Y₂O₃:Bi³⁺ (1%) were recorded in a transmission electron microscope at 200 keV, low current density and various temperatures. The quenching energy of the intrinsic luminescence of the various Y₂O₃:Ln³⁺ (Ln = Tb, Er and Tm) phosphors was found to be 0.25 eV. The intrinsic luminescence and the strongest spectral transitions of Ln³⁺ in these three phosphors exhibit similar temperature behaviour at temperatures > −50 °C, viz. a small increase of the spectral radiance upon increasing the temperature. Increasing the temperature beyond −50 °C led to complete quenching of the intrinsic luminescence at room temperature, whereas the radiance of the Ln³⁺ spectral transitions only decreased slightly. An extended Jablonski diagram for the energy transfer from the self-trapped exciton states in Y₂O₃ to the Ln³⁺ and Bi³⁺ ions is proposed. This diagram also indicates why Tb³⁺ is a better quencher of the intrinsic luminescence in Y₂O₃ than Er³⁺ and Tm³⁺. The intrinsic luminescence of Y₂O₃:Bi³⁺ largely overlapped with the blue Bi³⁺ emission band, which made an accurate analysis of its temperature behaviour impossible. Nevertheless, we concluded that upon increasing the temperature energy from Bi³⁺ ions at the C_3i sites is transferred to Bi³⁺ ions at C₂ sites. From the temperature behaviour of the 539 nm transition of the ²H_11/2 → ⁴I_15/2 manifold of Y₂O₃:Er³⁺ the activation energy for this transition could be determined: viz. 0.078 eV (623 cm⁻¹).